Disk-to-disk-to-tape (D2D2T) is an approach to computer storage backup and archiving in which data is initially copied to backup storage on a disk storage system and then periodically copied again to a tape storage system.
Disk-based backup systems and tape-based systems both have advantages and drawbacks. For many computer applications, it's important to have backup data immediately available when the primary disk becomes inaccessible. In this scenario, the time to restore data from tape would be considered unacceptable. Disk backup is a better solution because data transfer can be four-to-five times faster than is possible with tape. However, tape is a more economical way to archive data that needs to be kept for a long time. Tape is also portable, making it a good choice for off-site storage.
A D2D2T scheme provides the best of both worlds. It allows the administrator to automate daily backups on disk so he has the ability to implement fast restores and then move data to tape when he has time. The use of tape also makes it possible to move more mature data offsite for disaster recovery protection and to comply with regulatory policies for long-term data retention at a relatively inexpensive cost.
Disk-to-disk-to-tape is often used as part of a storage virtualization system where the storage administrator can express a company's needs in terms of storage policies rather than in terms of the physical devices to be used.